I just bought a Erlenmeyer Flask from a local brew shop. It was around 21 bucks, and doesn't have a brand name on it. I know the brew shop's supplier is LD Carlson and can assume pretty safely that is where they got this one. LD Carlson's web site does not specify if this is borosilicate glass...

I would like to boil a starter in it and throw it directly into an ice bath to cool. Obviously if this is not borosilicate glass this would be a bad idea. So does anyone know if LD Carlson's are, or if all E-flasks are made like this?

I bought a similar one from my LHBS ($18 for 2 liter). The owner said that it was not Pyrex, so to be careful about thermal shocks. I intend to treat mine like I would a carboy, adding boiled and chilled wort to it.

E-Flasks can be made with different materials. Pyrex and Kimax are 2 I know of. They should both be able to handle thermal shock pretty well, but nothing is a guarantee. When I used to make starters in E-Flasks, I would first put it in a bath of regular tap water (~55-60°F) for a few minutes and then an ice bath. Never had a problem.

If you're going to boil starter wort in them, get the anti-foam drops. Volcanos burn and make a huge sticky mess. I make my starters in a stock pot now and funnel it in to flasks for this reason.

If it doesn't say Pyrex or Kimax or Duran assume it is not borosilicate unless you can confirm that it is.

Also note that Pyrex is not always borosilicate, since Corning uses that branding on all of its kitchenware products many of which are not borosilicate. However labware labeled Pyrex is almost certainly borosilicate.

If it isn't borosilicate, I would return it since it offers no significant advantage over a reused mason jar, growler etc.